1. Field of the Invention
This invention concerns cast iron material and chilled cast iron.
2. Prior Art
As a means for improving the wear resistance at a specific surface of a cast iron article, there has been known a so-called remelting surface hardening treatment which comprises partially remelting only the surface of the cast iron article by a high energy souce such as TIG (Tungsten Inert Gas) arc, plasma arc or electron beam in a protection gas atmosphere such as an inert gas and rapidly cooling the remelted portion by the self-cooling performance of the article, thereby forming a chilled layer composed of a hard and dense chilled structure. According to the remelting surface hardening treatment, only the specific surface of the cast iron article can be hardened to thereby improve the wear resistance thereof with no change in the properties of the main portion thereof.
However, when the remelting surface hardening treatment is applied to gray cast iron which is most ordinary as a cast iron and, particularly, if the scanning rate of the high energy of source to the cast iron article is high, there has been a problem of blowhole generation in the remelted portion. Since graphite in the gray cast iron matrix constitutes individual cells in the form of relatively coarse flake graphite involving the gas therein, when the graphite is exposed to a highly activated state under heating by the high energy source, it chemically reacts mainly with the oxygen in the gas into carbon monoxide or the like, also under the effect of the oxygen existent in the cast iron matrix or the oxygen intruding from the atmosphere while breaking through the shielding of the protection gas. The gas thus resulted produces gas bubbles in the molten iron to rapidly cool and coagulate the molten iron and trapped to remain as blowhole in the remelted portion. Such blowholes appear at the surface of the cast iron article when machining such as grinding is applied to the remelted portion, to prevent the improvement in the wear resistance of the remelted portion, as well as impair the appearance of the article or the worsen degree of surface finishment for the article.
In order to prevent the blowhole generation, special covers have been used for covering the remelted portion or the flow rate of the protection gas has been increased in order to form a sufficient protection shielding to the remelted portion. However, since the formation of carbon monoxide caused by the oxygen existent in the cast iron cannot be prevented by these methods, blowhole generation cannot completely be depressed and they bring about an additional problem of increasing the cost for the remelting surface hardening treatment.
There has also been known effective, as another method for preventing blowhole generation, to preheat the cast iron article to a temperature of about 400-500.degree. C. prior to the remelting treatment. However, this method requires additional step and apparatus for the preheating treatment, which increases the energy consumption and treatment cost, as well as lowers the self-cooling performance of the cast iron article due to the preheating treatment for the cast iron article prior to the remelting treatment thereby failing to provide a sufficient improvement in the hardness of the thus obtained remelting chilled texture.
It may further be considered, as a further method of depressing the blowhole generation, to reduce the scanning rate of the high energy source to the cast iron article, but this increases the amount of heat intruding to the cast iron article, making it difficult to obtain a sufficient cooling rate only with the self-cooling of the cast iron article and requires an additional forcible cooling means.
As a still further method of depressing the blowhole generation, it has been recommended to decrease the specific surface area of individual graphite grains in contact with the iron matrix in view of the graphite shape in the matrix such as eutectic graphite, chunky graphite or spherical graphite (for instance, in Japanese patent Laid-Open Nos. 85,926/1982 and 149,420/1982, and U.S. Pat. No. 4,000,011). However, these forms of graphite involve the respective problems as described below.
Referring at first, eutectic graphite is generally formed by using titanium. Although blowholes can significantly be decreased, large blowholes are resulted in places. This is attributable to the inclusion of dross increased at the surface of the molten iron caused by the addition of titanium. Further, titanium remains upon casting in the molten iron as a reversing material and this means that an exclusive casting line has to be used or the reversing agent has to be checked in a great number of steps.
Referring then to chunky graphite, the cast iron containing such a shape of graphite with the sphericalizing rate from 20 to 70% is generally called as vermicular cast iron, although having high C.E. (Carbon Equivalent) value and not belonging to the gray cast iron. A molten iron treating agent usually based on magnesium is used to provide a significant effect against the blowhole generation. However, since the magnesium is used as the treating agent, molten iron is splashed and white flash or smoke is resulted due to the evaporation of magnesium during handling of the molten iron to bring about problems in safety or sanitary point of view. Further, since the sphericalizing rate is not stable, shrinkaging property is dispersed. Accordingly, a larger amount of riser than required is necessary in order to obtain crude materials at high quality, to worsen the yield and increase the cost. Further, since the magnesium in the matrix inhibits the flowability of the molten pool upon applying the remelting treatment, a desired hardening depth cannot be secured unless the amount of intruding heat is increased, to provide an adverse effect on the energy saving. Further, since the specific surface area is smaller as compared with that of the A-type graphite, not-melted residue of graphite is resulted upon remelting, making it difficult to form a uniform remelted chilled layer.
In the case of spherical graphite cast iron using magnesium as the sphericalizing agent, the similar disadvantage to that in the chunky graphite is resulted and the situation is further worsened. Further, in view of the cost, since magnesium has to be incorporated by from 0.040 to 0.080% and the C.E. value is as high as from 4.5 to 5.0, larger amount of C and Si has to be used to increase the cost. The C.E. value used herein is defined as follows: EQU C.E. value=Total carbon+1/3 silicon wt%
In this case, 4.3 of the C.E. value indicates eutectic, and not more than 4.3 of the C.E. value indicates hypoeutectic. Further, the generation of slags after the molten iron treatment is remarkable and the amount of the slag catcher used and the number of slag-removing steps at high temperature is also increased.
In the case of using calcium as the sphericalizing agent, light and less soluble calcium readily tends to float on the upper portion of the molten metal and react with oxygen into slags and the yield is as low as 20-50%.